Prompt with escape sequences breaks backspace handling

[egranata]

Imagine doing something like this

        prompt.append_sprintf("\x1b[38;2;205;205;0m%s\x1b[0m$ ", cwd.c_str());

in order to (in this case) color the CWD yellow in a shell prompt.

If linenoise is fed this prompt text, it will do an incorrect calculation of how many characters to move the cursor by in a refreshSingleLine() call after, e.g., pressing BACKSPACE - assuming that it needs to backspace the non-printable escape sequence characters